The present invention relates to a semiconductor device and a method for manufacturing it, and more particularly to a method for selectively growing a flat, smooth silicon epitaxial layer on a single crystal semiconductor substrate having an insulating film at predetermined locations on its major surface, and to a semiconductor device having circuit elements formed in such a silicon epitaxial layer.
For a semiconductor device which must have a high density and a high operation speed, the problems of an isolating structure positioned between active elements and of a parasitic capacitance become important. Heretofore, circuit elements have been isolated from each other, for instance, by a method of partially oxidizing a semiconductor substrate in which the circuit elements are formed and by a method of forming isolation regions by making use of an impurity.
However, these known methods are not suitable for high density circuit integration because the gap distances between adjacent active regions must be made relatively large.
In addition, a parasitic capacitance is produced by a PN junction between the bottom of an active element in a semiconductor substrate and the same semiconductor substrate. The capacitance creates a problem in increasing the operational speed.
The above-mentioned problem, can be solved by forming a silicon epitaxial layer (silicon monocrystalline layer) on an insulating film or an insulating substrate and by forming an element in this silicon epitaxial layer. To that end, heretofore, the technique of SOS (Silicon On Sapphire) has been available. However, the junction formed between a sapphire substrate and a silicon layer thereon is a heterojunction. Therefore, many lattice defects are present along the boundary face of the junction and those defects become a cause of leakage current.
On the other hand, a method of converting polycrystalline silicon into a monocrystalline silicon by means of a laser was proposed by M. W. Greis et al. in Applied Physics Letters Vol. 35, July 1979, pages 71 to 74. In more particular, a grating is formed periodically on silica (SiO.sub.2), and polycrystalline silicon is deposited on the grating through a CVD (Chemical Vapor Deposition) process. Thereafter, as a result of melting by laser irradiation, recrystallization occurs from the grating into a monocrystalline silicon. However, this method involves many problems in a practical application such as problems with crystal grain sizes, a degree of conversion into a monocrystalline silicon, generation of lattice defects, etc.
The method of converting polycrystalline silicon into a monocrystalline silicon by laser irradiation was also proposed by Masao Tamura et al. in the Japanese Journal of Applied Physics, Vol. 19, No. 1, January 1980, pages L23 to L26. According to this method, a window is opened in a silicon dioxide film on a silicon single crystal substrate and polycrystalline silicon is deposited over the entire surface, through a CVD process. Thereafter, by laser irradiation recrystallization is attempted from the silicon substrate to convert the polycrystalline silicon on the silicon dioxide film into a monocrystalline silicon. However, this method also involves problems such as a degree of conversion into a monocrystalline silicon, crystal defects on the insulating film, etc. narrower, the method is not practical, since a recess having a depth of the order of the thickness of the silicon dioxide film is formed on the surface of the converted single crystal. Thus, the laser irradiation technique has not matured into a practically useful technique.
On the other hand, in the Journal of The Electrochemical Society: SOLID-STATE SCIENCE AND TECHNOLOGY, May 1973, pages 664 to 668 P. Rai-Choudhury et al. have proposed a method for selectively growing a silicon epitaxial layer (a monocrystalline layer). According to this method, silicon tetrachloride (SiCl.sub.4) is grown under a reduced pressure (7.times.10.sup.-3 atm.). However, in this method making use of SiCl.sub.4, the growing temperature of SiCl.sub.4 becomes higher than 1100.degree. C. Consequently, a silicon epitaxial layer having a good crystallinity cannot be obtained. In addition, because of the high growing temperature, stresses existing between the insulating film and the silicon epitaxial layer became large. Therefore, a leakage property of the elements produced in this epitaxial layer deteriorates. Furthermore, since the growing temperature is high, a redistribution of an impurity in the epitaxial layer is liable to occur. Hence, the operating characteristics of the produced elements flucturate. Thus, this method is also poor in practical applications.
Another method for selectively growing a silicon epitaxial layer is proposed by R. K. Smeltzer in the Journal of the Electrochemical Society: SOLID-STATE SCIENCE AND TECHNOLOGY, December 1975, pages 1666 to 1671. According to this method, a groove of 10-20 .mu.m width and about 100 .mu.m depth is formed in a silicon substrate. An attempt is made to have a selective growth of a silicon epitaxial layer in the groove to fill up the groove. In this method, as a source of silicon, SiH.sub.4, SiHCl.sub.3, SiH.sub.2 Cl.sub.2 or SiCl.sub.4 is employed, and the silicon single crystal is grown at atmospheric pressure while adding HCl to the silicon source. Since, growth takes place at atmospheric pressure, there is a large pattern shift and facets are produced on a crystalline surface, resulting in an unevenness of the surface. In addition, in the method of growing at an atmospheric pressure, crystallization is not smooth and hence a uniform film thickness cannot be obtained.
Accordingly, even if an epitaxial layer were to be grown on an insulating film above a silicon substrate by making use of this method, it would be impossible to form an element in that epitaxial layer.
As described above, if a silicon epitaxial layer (a silicon monocrystalline layer) is grown on an insulating film according to the prior art techniques, a circuit element is difficult to form in that layer. Especially it is impossible to produce an insulated gate field effect transistor (hereinafter abbreviated as "IGFET"), for which a surface condition is very important, in a silicon epitaxial layer grown according to the prior art technique.